Electromagnetic levitation involves the use of a radio frequency field acting on a sample having some degree of conductance. The field induces a current in the sample which in turn reacts to repel the inducing field. The field is generated by an alternating current power source connected to specially designed coil, which is fabricated from water cooled copper tubing. The coil is connected to a single RF amplifier, with the coil comprising two connected coaxial portions, with one portion being wound in a direction opposite to the other. The field of one portion is 180 degrees out of phase with the other portion and produces a null point along the axis where the two fields are equal in strength, allowing levitation of a specimen near the null point.
The power required to levitate a sample also causes the sample to be heated to a degree dependent on the eddy currents and hence on the field strength or coil current at a given frequency. Many experiments have been proposed and conducted in which a levitated sample is heated and cooled, for reasons more fully set forth below.
Electromagnetic levitation has traditionally involved numerous problems and tradeoffs, which have not been adequately addressed or resolved by prior art methods. The levitator has involved the use of a single, large and bulky power source, which is connected at some distance from the coil. Due to substantial line losses, most of the power is converted into heat necessitating the use of water cooling to prevent the leads and the coil from melting. As a result, efficiencies of approximately 30% or less are achieved.
The coil configurations of prior art coils have involved compromise. To produce optimum heating, a solenoid-type coil may be employed, but only at the sacrifice of levitation forces. A crossed or opposite phase coil is more efficient for levitation, but the forces available to retain the sample at angles to the axis between the coils is very weak. As a practical matter, it is usually very simple to levitate and melt a sample. When the sample is cooled by a reduction in power, however, the electromagnetic field may not be sufficient to hold the sample in a stabilized levitated position. Also, lack of restraining forces away from the axis may allow a molten sample to escape under the influence of gravity. The positioning and heating capabilities of such devices cannot be independently controlled.
Electromagnetic induction techniques have been used to levitate, position, and melt conductive or metal specimens. Various applications include metal shaping, welding, themophysical property measurements, continuous casting, non-contact supercooling, and melt processing in a containerless environment. The latter is a very important application, since it allows melting of materials which are highly reactive at high temperature, and which would be contaminated by a crucible. High temperatures (4000K) are obtainable and vacuum or inert gas environments can be used to investigate such highly reactive materials.
It would be desirable to provide independent positioning and heating/cooling control in an electromagnetic induction device without the use of separate heating devices. It would also be desirable to provide a levitation device with a more stabilized levitation location, i.e., to equalize the repulsive forces surrounding the levitated specimen and provide enhanced stability and uniformly distribute the flow of eddy currents in the sample. Another objective would be to decrease the size and line losses attributable to present day power supplies, which would result in greatly improved efficiencies, and would eliminate the need for water cooling and the difficult shaping of copper tubing.